1. Field of the Invention
This invention relates to electric fuses. More particularly it relates to means for establishing a seal between a metal terminal plug and the tubular electric fuse casing into which the plug is inserted.
2. Description of the Prior Art
It has for a long time been recognized that it is desirable for an electric fuse to be designed such that the fuse casing and end terminals cooperate with one another to prevent the loss of the pulverulent arc-quenching filler contained within the fuse casing, and, to prevent the escape of products of arcing when the fuse "blows" in response to overcurrent conditions. In fuses where the casings are closed by large metal plugs, having a relatively large heat absorbing capacity, the problem the fuse designe faces is how to prvent the escape of such products of arcing and the arc-quenching filler through the fine annular gap formed between the outer peripheral surface of the plug and the inner surface of the fuse casing.
One approach to solving this problem is to maintain close tolerances between the plugs and the inner surface of the fuse casing. Maintaining sufficiently close tolerances to solve this problem would substantially increase the cost of production of such a fuse. If the tolerances of the plugs and casing were set so the define a tight fit into the casing, considerable undesirable stresses could be set up in the casing materials. A number of approaches are known in the prior art for sealing electric fuses whose casings are closed by a pair of metal terminal plugs. These approaches include the use of annular seals, such as o-rings, and the use of annular seals formed from a settable sealing material injected into annular grooves provided in the terminal plugs through communicating with the grooves formed in the sidewalls of the tube casings.
Other known approaches have been to "butter" on the outside of the terminal plug, a sealing material such as RTV silicone rubber adhesive sealant. Another approach has been use of an epoxy material to jacket the ends to attempt to effect the seal.
U.S. Pat. No. 2,639,350, Electric Fuse, shows an electric fuse having a terminal cap of the "ferrule type" which fits over the outside of the ends of the tubular casing. The fuse tube itself is provided with one or more annular grooves and yieldable sealing rings are located in the annular grooves to effect a seal.
U.S. Pat. No. 2,837,614, Protectors for Electric Circuits, discloses an electric fuse having annular grooves in the plug terminals of the fuse, and, provides o-ring in those grooves. This patent also notes that a smooth finish on the inner surface of the casing of the fuse is desirable to obtain a good seal.
U.S. Pat. No. 3,250,879, Electric Fuse Comprising Plug Terminals Having an Improved Seal and Pinning Means, discloses an electric fuse having a plug-type terminal having a plurality of annular grooves therein for forming a seal with the fuse casing. An appropriate sealing medium is introduced through an opening in the fuse casing sidewall which fills the annular grooves to effect a seal. The seal material also extends into the openings in the lateral side walls of the fuse casing to effect a "pinning" of the plug terminal to the fuse casing.
U.S. Pat. No. 4,044,326, Hermetic Seal between Telescoping Cylinders of a Fuse Housing, discloses a seal arrangement for a plug type terminal which is a achieved by a combination of a resilient gasket member and a settable adhesive filling an annular space between the resilient gasket member and a second gasket member or a structural flange or the like.
In the real world of fuse manufacture it has been found that the inner lateral surface of the ends of a fuse casing commonly contain a number of irregularities which have been found to make o-ring seals not capable of effecting a consistently reliable seal between the plug terminal and the fuse casing. Various other techniques of injecting settable adhesives, sealants and epoxy have been found to be time consuming, require long cure times, and to be messy with the injected material often times finding it way into the interior of the fuse and or squirting out the outer end of the fuse. Further, injection techniques wherein the injection is carried out through openings formed in a lateral surface of the fuse casing require comprise of the structural integrity of the casing and lead to a potential vent path for the high pressure products of arcing from the fuse casing, if the seal is not perfectly established.
The object of the present invention is to provide an effective, low cost, clean and fast arrangement for establishing a reliable, high integrity seal between an end terminal block and the interior of the fuse tube of an electric fuse into which the block is received.
It is another object of the present invention to provide such a seal wherein the parts may be handled immediately after injection of the seal and wherein the injection technique does not aloow objectionable quantities of the injected sealant material to pass into the interior of the fuse casing nor out the axial outer end of the fuse casing.
Other objects and advantages of the present invention will become apparent from an examination of the drawing and the accompanying description.